I have always had large pupils to the extent that when I go to the eye doctor they don't even have to use dilating drops to examine the back of my eye. It is absolutely possible for you to have dilated pupils without being on drugs... in fact most drugs of abuse cause constricted pupils (not all but most). I think part of the problem is that many people with bipolar disorder also have a co-occurring substance abuse disorder. That may be why your doctor was so questioning of you. If you tell me you're not using any illegal drugs, I believe you. All you can do is maintain your innocence. Good luck with this situation and I'm sorry you're having such a difficult time. Hang in there!!